Alyson Hannigan, 'Buffy the Vampire Slayer'

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Then: Willow started out as the shy, nerdy, lovable witch, but, as shy, nerdy, lovable people tend to do, she went all vengeful villain in Season 6, when her girlfriend was killed and her grief fueled her dark power. You could tell by her hair. Now: Hannigan has never strayed far from TV. Her show "How I Met Your Mother" is in its eighth season on CBS. She's also returned to the "American Pie" series as the famous flutist in the latest installment.

Then: Willow started out as the shy, nerdy, lovable witch, but, as shy, nerdy, lovable people tend to do, she went all vengeful villain in Season 6, when her girlfriend was killed and her grief fueled her dark power. You could tell by her hair. Now: Hannigan has never strayed far from TV. Her show "How I Met Your Mother" is in its eighth season on CBS. She's also returned to the "American Pie" series as the famous flutist in the latest installment. (Getty Images/WB)

Then: Willow started out as the shy, nerdy, lovable witch, but, as shy, nerdy, lovable people tend to do, she went all vengeful villain in Season 6, when her girlfriend was killed and her grief fueled her dark power. You could tell by her hair. Now: Hannigan has never strayed far from TV. Her show "How I Met Your Mother" is in its eighth season on CBS. She's also returned to the "American Pie" series as the famous flutist in the latest installment.